Settings

The settings are stored under
Preferences / Package Settings / Resize Group With Keyboard / Settings - Default.
In order to change the default settings, copy the settings to Settings - Default
found under the same menu, and edit their values.

Setting the resize delta

The resize delta is set to 0.01 by default, similar to 1 % of the entire application window.
Change this to a greater or a smaller value in order to resize the groups accordingly.

{"resize_delta_default":0.01}

Per-direction resizing

Optionally, the resize deltas can be set explicitly for each of the directions.
This can be handy you want different resizing with e.g. Up/Down and Left/Right.
For example: